The invention relates generally to solid-state image detecting apparatus and, more particularly, to infrared image detecting apparatus including two stacked image detector arrays and methods for making same.
Solid-state image detecting apparatus is well known. Such detecting apparatus includes a plurality of photosensitive image detector elements. In accordance with well known principles, a charge is generated within each detector element as a function of the amount of electromagnetic radiation that is incident thereon. The plurality of detector elements are typically integrated in a semiconductor substrate as either a linear image detector or as an area image detector. The linear image detector includes a narrow line of detector elements that can be used to obtain a two-dimensional image by mechanically scanning the object or scene to be imaged, such as by use of a rotating mirror.
The area image detector is a two-dimensional array of detector elements onto which is projected an image of the object or scene to be imaged. In cases where the two-dimensional array is small, a two-dimensional image can be obtained by a step-and-stare technique, known in the art, in which, by use of additional optical and mechanical means, successive portions of the object or scene to be imaged are projected onto the small array. When the two-dimensional array of detector elements is sufficiently large, the array can be employed as a staring array, such that the entire object or scene to be imaged is projected onto the array. In general, subject to cost considerations, it is desirable to provide a two-dimensional array of detector elements with as large an area as possible in order to simplify and enhance the imaging process.
The spectral band of infrared radiation is generally divided into short (SWIR), middle (MWIR), and long (LWIR) ranges, respectively corresponding to wavelength ranges of 1-3 .mu.m, 3-5 .mu.m, and 8-14 .mu.m. The efficacy of detector elements for one or another of the IR wavelength ranges varies with the composition of the detector elements. For example, MWIR detector arrays with dimensions of 128.times.128 pixels have been successfully fabricated with InSb and HgCdTe detector elements. The dimensions of 128.times.128 pixels are relatively small and, depending on the application, would likely be implemented as a step-and-stare array.
Further, LWIR detector arrays having dimensions of 480.times. 4 pixels have been successfully fabricated with HgCdTe detector elements, an array with such dimensions being implemented as a scanning array. Disadvantageously, the production techniques for InSb and HgCdTe detectors are unique and are undergoing further development. Further, the starting materials for such detectors are expensive and the substrates are small. As a result, the fabrication yields of such arrays are very low and their production costs are very high. The difficulties currently encountered in producing functional arrays using such detector compositions necessarily limits the dimensions of the arrays being produced. An advantage of InSb and HgCdTe detectors is their very high quantum efficiency, which is on the order of 70-90%.
Other IR detector compositions are also known. In general, silicide Schottky barrier detectors have been successfully implemented in SWIR, MWIR, and LWIR detector arrays PdSi, PtSi, and IrSi Schottky barrier detectors are examples of such detectors. PtSi Schottky barrier detectors have been successfully implemented in SWIR and MWIR detector arrays. For example, PtSi Schottky barrier cameras having dimensions of 512.times.512 pixels are commercially available for MWIR applications. Larger arrays of PtSi detector elements, having dimensions on the order of 1024.times.1024 pixels, are under development. Arrays having such dimensions are typically implemented as staring arrays. An example of image detecting apparatus including PtSi Schottky barrier detector elements, and a method for making same, is described in "160.times. 244 Element PtSi Schottky-Barrier IR-CCD Image Sensor" by Kosonocky et al., IEEE Transactions on Electron Devices, Vol. ED-32, No. 8, August 1985, which is incorporated herein by reference.
IrSi Schottky barrier detector arrays have been found to be suitable for LWIR applications. An example of an IrSi Schottky barrier detector array is disclosed in "IrSi Schottky-Barrier Infrared Image Sensor" by N. Yutani et al., Proceedings of the 1987 International Electron Devices Meeting, IEEE Paper No. CH2515-5/87/0000-0124, which is incorporated herein by reference. Characteristics of IrSi detectors implemented for MWIR applications are also described in "High Emission Efficiency Iridium Silicide for MWIR Sensor" by D. Lang et al., Proceedings of the 1989 IRIS Detector Specialty Group Meeting, vol. I, which is incorporated herein by reference.
One advantage of the silicide Schottky barrier detectors is that, generally, they are fabricated on a standard silicon fabrication line using conventional integrated circuit fabrication processes and equipment. The use of conventional fabrication processes contributes to the ability to fabricate arrays having relatively large dimensions, e.g., 512.times.512 pixels and larger. Further, the use of conventional fabrication processes results in both a reduced cost of fabrication and a higher yield.
A disadvantage of silicide Schottky barrier detectors is their relatively low quantum efficiency, which is on the order of 0.1-3% for PtSi and IrSi detectors, depending on wavelength. Such detectors also have very low optical absorption. However, arrays with such detectors can be constructed to have dimensions sufficiently large to be implemented as staring arrays. As a result, the relatively long integration period permitted in a staring array at least partially compensates for the low quantum efficiency.
A further disadvantage of silicide Schottky barrier detector arrays, and semiconductor devices generally, is that even though they can be produced with relatively large dimensions and acceptable production yields, the production yield decreases as dimensions, such as array dimensions, are further increased or feature sizes are decreased.
A disadvantage shared by all of the above described detector technologies is the difficulty with which redundancy is implemented. In this context, redundancy refers to providing means for compensating for one or more nonfunctioning detector elements. It is noted that a detector element may be rendered nonfunctional not only by defects or failures in the detector element itself, but also by defects or failures in external circuitry coupled to the detector element, e.g., bias or readout circuitry. One redundancy technique known in the art for compensating for a nonfunctioning detector element or elements includes mechanically moving the detector array to sample a pixel of an image by more than one detector element. Thus, disadvantageously, in accordance with this technique, additional mechanical apparatus is required for moving the detector array. In accordance with another known redundancy technique, the outputs of pixels surrounding a nonfunctioning element are averaged or substituted for the nonfunctioning element in order to provide image information corresponding to the nonfunctioning element.
As known in the art, large detector arrays, e.g., 512.times.512 pixels and larger, are frequently divided into subarrays for the purpose of facilitating the readout of image information. In accordance with such division into subarrays, readout circuitry is separately coupled to the respective subarrays, so that the pixel readout rate can be reduced, thereby reducing noise. While the practice of dividing a detector array into subarrays provides advantages with respect to reading out image information, it results in the disadvantage that a defect or failure incurred in the readout circuitry for a particular subarray likely renders the entire subarray nonfunctional. None of the above described redundancy techniques, or any other prior art techniques of which the inventor is aware, adequately compensates for a nonfunctioning subarray.
It is therefore an object of the present invention to provide an IR image detecting device that is not subject to the aforementioned problems and disadvantages of the prior art.
It is another object of the present invention to provide an IR image detecting device that comprises a large staring array and that can be fabricated with a high production yield.
It is a further object of the present invention to provide an IR image detecting device that incorporates a redundancy technique that compensates for a nonfunctioning subarray.